Optical measurements making it possible to characterize optical properties of samples are widespread. Measurements based on the detection of a signal backscattered or reflected by a sample illuminated by a light beam may be cited in particular. These may entail, for example, Raman spectrometry, fluorescence imaging or diffuse reflectance spectrometry.
Diffuse reflectance spectrometry, commonly referred to by the acronym DRS, consists in utilizing the light backscattered by a scattering object subjected to pointlike illumination. This technique is described in document EP2762064 and in publication Sorgato V. “Non-contact quantitative diffuse reflectance spectroscopy”, Proceedings of SPIE Vol. 9538, 95380U (2015), referred to hereinafter as “Sorgato 2015”. These documents describe a probe including a central optical fibre, connected to a light source and intended to direct a thin beam of light onto a sample, in particular a skin sample. Optical fibres positioned around the central optical fibre, termed detection fibres, collect optical radiation backscattered by the sample. Means for spectrally analysing the backscattered radiation allow optical properties of the sample to be estimated, in particular optical properties relating to scattering and to absorption. The probe may be brought into contact with or remote from the sample to be characterized. This technique makes it possible to quantitatively characterize these optical properties, but it is a local characterization. The optical properties are determined within an elementary volume of the sample, this elementary volume being dependent on the geometry of the probe.
It is possible to take DRS measurements at various discrete positions on a sample, according to a spatial mesh, so as to obtain a two-dimensional characterization of the optical properties of the sample. Such an approach is presented in publication Nichols B S et al. “A Quantitative Diffuse Reflectance Imaging (QDRI) System for Comprehensive Surveillance of the Morphological Landscape in Breast Tumor Margins”, PLoS ONE 10, 2015, referred to hereinafter as “Nichols 2015”. The device and method described in this publication make it possible to characterize optical properties according to a discrete two-dimensional mesh comprising 49 points distributed regularly in a matrix arrangement.
As an alternative to DRS, multispectral imaging, referred to by the acronym MSI, makes it possible to characterize an optical property of a sample in two dimensions. Publication Bjorgan A, “Estimation of skin optical parameters for real-time hyperspectral imaging applications”, J. Biomed. Opt. 19(6), 066003, Jun. 4, 2014, referred to hereinafter as “Bjorgan 2014”, is one example thereof. This publication describes the use of images of a sample, formed in various spectral bands, in order to evaluate a two-dimensional spatial distribution of the absorption coefficient of the sample, this being done in the various spectral bands. The evaluation is based on a value of the reduced absorption coefficient that is determined beforehand on the basis of data that are available in the literature. Since it is based on images, this method may be applied across a wide field of observation, and provides both good spatial and spectral resolution. However, as described below, the inventors have demonstrated that such a method may result in a substantial measurement error when it is sought to obtain a precise quantification of the absorption coefficient.
The inventors have sought a method making it possible to quantitatively characterize the scattering and absorption optical properties of a sample, providing an extended field of observation, so as to quickly obtain a two-dimensional spatial distribution of these optical properties. This method implements simple and inexpensive devices and is robust enough to be applied outside a research laboratory, for example in an operating theatre or in an analytical laboratory.